1. Field of the Invention
The present invention relates to a caption line finding circuit for a video signal processor, and in particular to an improved caption line finding circuit for a video signal processor capable of more correctly finding a caption line, in which a caption data is contained, among pictorial signals.
2. Description of the Conventional Art
A caption means that a certain character is displayed on a screen of a television after caption data contained in an encoded video signal is decoded.
Referring to FIG. 1, a conventional caption line finding circuit for a video signal processor includes a synchronous tip clamp 1 for clamping a video signal VID inputted thereto at a pulse level of a horizontal synchronous signal, a data slicer 2 for slicing a video signal into small data units as a direct current voltage is inputted to a first input terminal thereof and a video signal is inputted to a second other input terminal thereof, a line finder 3 for finding a scanning line, in which a caption data is contained, from a video signal outputted from the synchronous tip clamp 1 using vertical and horizontal frequencies inputted thereto, a logic unit 4 for determining an appropriate sampling rate for recovering an encoded caption data after being enabled by the output signal of the line finder 3, a digital/analog converter 5 for converting the output signal of the logic unit 4 into a digital/analog signal and for outputting a direct current voltage which can be a slicing level, a filter 6 for filtering the output signal of the D/A converter 5 and for outputting the filtered signal to the data slicer 2, and a sampling unit 7 for sampling the video signal outputted from the data slicer 2 in accordance with a control signal of the logic unit 4. Here, a dot clock is applied to the line finder 3 and the logic unit 4, respectively, for a sampling operation, and an automatic/manual control signal AUTO/MAN is applied to the logic unit 4 in order to control the level of a slicing operation.
The operation of the conventional caption line finding circuit for a video signal processor will now be explained with reference to FIGS. 2 and 3.
The caption line finding circuit is controlled by a microprocessor (not shown) provided at a predetermined location in a television. The video signal is inputted to the synchronous tip clamp 1. At this time, the synchronous tip clamp 1 clamps the pictorial signal, inputted to the synchronous tip clamp 1, to a horizontal synchronous tip level. The output signal of the synchronous tip clamp 1 is inputted to the second input of the data slicer 2 and is converted into a voltage pulse of a logic level. Here, the first side of the data slicer 2 receives a DC level voltage. In addition, the logic unit 4, which is enabled by the output signal of the line finder 3, selects an appropriate sampling rate in order to recover the caption data encoded in accordance with a control signal of a corresponding program. Thereafter, the sampling unit 7 samples the signal outputted from the data slicer 2 in accordance with a control signal of the logic unit 4. The sampled data is decoded by a control circuit (not shown) and is displayed on the screen of a television in certain characters. Meanwhile, the D/A converter 5 converts the output signal of the logic unit 4 into a D/A signal and outputs a DC voltage level, which is a slicing level, to the filter 6. Thereafter, the filter 6 filters the output signal of the D/A converter 5 and outputs the filtered signal to the data slicer 2.
The above described operation will now be explained with reference to FIG. 3.
When a video signal is inputted, a certain odd or even field is selected, and the pulse number of a horizontal synchronous signal is set as 17 (seventeen), and a vertical synchronous signal is detected. Thereafter, whether the current signal inputted thereto is an odd field or an even field is tested. As a result of the test, if the current pictorial signal is not a desired field, a step which detects a vertical synchronous signal V is repeated until the desired one is detected. However, as a result of the test, if the current field is a desired one, the pulse number N of the horizontal synchronous signal is counted, and whether a clock run-in signal is present or not is checked. As a result of the check, if the clock run-in signal is not present, the pulse number N of the horizontal synchronous signal is increased by 1 (one). If the pulse number N is within a range of 15&lt;N&lt;20, the step which detects a vertical synchronous signal V is performed. If the pulse number N is not within a range of 15&lt;N&lt;20, the step which detects a vertical synchronous signal V is performed again after the current field is changed to another field, and the pulse number N is set as 15. However, if the clock run-in signal is present, the data is sampled and loaded to a certain shift register after the sample clock is synchronized with the clock run-in signal. If the start bit is "001" which can be one of the caption data start condition in the loaded data, the loaded data is recognized as a caption data, so the encoded caption data is decoded and outputted. Meanwhile, if the start bit is not "001," a condition that there is no caption data is recognized, so the step which increases N by 1 (one) and detects the vertical horizontal signal V is performed.
However, the conventional caption line finding circuit for a video signal processor has disadvantages in that a tape may be elongated due to the temperature variation, and a horizontal scanning line in a video signal can be missed, resulting in the caption line not being found.